Age Calculation Reference (2026): Methods and Edge Cases

Every row is produced by running AgeCalcTool's own calculator function directly (not estimated by hand): whole years first, then whole months, then leftover days, borrowing a month's worth of days from the calendar month immediately before the target date whenever the day-of-month subtraction goes negative. Total days is a separate, independent count of elapsed calendar days between the same two dates. Because this is deterministic date arithmetic built on the Gregorian calendar rather than a survey or market statistic, there is no external primary source to cite for the results themselves; the method is the Gregorian leap-year rule (a year is a leap year if divisible by 4, except century years, which must be divisible by 400) applied through each browser's native date object. This page was last checked and regenerated on July 2, 2026, and will be refreshed if the underlying calculator logic changes.

Leap-day births (Feb 29): a person born on a leap day has a birthday that does not exist in most years. The calculator treats Feb 28 as the effective birthday in non-leap years, so age still advances on schedule, and the extra day resolves itself the next time Feb 29 occurs.

The 2100 century exception: under the Gregorian rule, century years are leap years only if divisible by 400. The year 2000 was a leap year; 2100 will not be. A person born Feb 29, 2004 checked against Feb 28, 2100 correctly resolves to 95 years, 11 months, 30 days, not a full 96 years, because Feb 29, 2100 will not exist.

Month-end borrowing: when a birth date falls near the end of a long month (for example the 31st) and the target date is early in a month two calendar months later, the day count has to borrow from the month in between. The calculator borrows the correct number of days for that specific month (28, 29, 30, or 31), so results stay accurate across months of different lengths.

Same-day and day-before checks: on the exact anniversary of a birthdate the result is a whole number of years with 0 months and 0 days; one calendar day earlier, the result correctly shows 11 months and roughly a month's worth of days rather than rounding up early.